Bottom Line:
In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method.Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects.Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects.

ABSTRACTThe swelling that occurs in uranium dioxide as a result of radiation-induced defect ingrowth is not fully understood. Experimental and theoretical groups have attempted to explain this phenomenon with various complex theories. In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method. Based on their resemblance to experimental data, the simulation results presented here show that fission induces only oxygen Frenkel pairs while alpha particle irradiation results in both oxygen and uranium Frenkel pair defects. Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects. It is shown that obstruction type Frenkel pairs are responsible for both fission- and alpha-particle-induced lattice swelling. Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects. Additionally, at high concentrations, some of the obstruction type uranium Frenkel pairs formed diatomic and triatomic structures with oxygen ions in their octahedral cages, increasing the slope of the linear dependence.

pone.0134500.g005: Günay potential results for variations in numbers of remaining and created FPs versus uranium IFPs.The inset was taken from Ref. 28 and shows the experimentally determined concentrations of uranium and oxygen FPs created, as functions of dose.

Mentions:
The number of uranium IFPs was varied from 0 to 30, which was a large enough range to demonstrate the saturation stage using both Yakub and Günay potentials. In contrast to UO2 with oxygen IFPs, recombination of some of the uranium vacancies and interstitials during the equilibration step resulted in the creation of distortion and obstruction type uranium and oxygen FP defects. Figs 5 and 6 show how the number of surviving (remaining uranium IFPs) and created (uranium and oxygen) FPs varied with the number of uranium IFPs. These results were obtained from the <110> direction, as shown in Fig 2. The numbers of all types of defects increased as the number of uranium IFPs increased, eventually reaching saturation values. Distortion type oxygen defects were the most numerous and showed a sharp increase, quickly achieving a saturation value. A similar behavior was observed by Turos et al. [28] in their experimental study on radiation defects in UO2. For comparison, the insets in Figs 5 and 6 show how U and O defect concentrations vary with increasing implantation dose. This indicates that the large difference between the steering forces of U and O rows is responsible for the higher saturation value observed for oxygen ions than uranium ions. Steering force arises from differences in the masses and mobilities of U and O ions. It is known that O ions are naturally more mobile than U ions and that they have a liquid-like self diffusion coefficient at temperatures approximately 20% lower than the melting temperature of UO2 which is a property of a superionic conductor. Therefore, oxygen defects are more easily created during uranium IFP recombination, even at 300 K.

pone.0134500.g005: Günay potential results for variations in numbers of remaining and created FPs versus uranium IFPs.The inset was taken from Ref. 28 and shows the experimentally determined concentrations of uranium and oxygen FPs created, as functions of dose.

Mentions:
The number of uranium IFPs was varied from 0 to 30, which was a large enough range to demonstrate the saturation stage using both Yakub and Günay potentials. In contrast to UO2 with oxygen IFPs, recombination of some of the uranium vacancies and interstitials during the equilibration step resulted in the creation of distortion and obstruction type uranium and oxygen FP defects. Figs 5 and 6 show how the number of surviving (remaining uranium IFPs) and created (uranium and oxygen) FPs varied with the number of uranium IFPs. These results were obtained from the <110> direction, as shown in Fig 2. The numbers of all types of defects increased as the number of uranium IFPs increased, eventually reaching saturation values. Distortion type oxygen defects were the most numerous and showed a sharp increase, quickly achieving a saturation value. A similar behavior was observed by Turos et al. [28] in their experimental study on radiation defects in UO2. For comparison, the insets in Figs 5 and 6 show how U and O defect concentrations vary with increasing implantation dose. This indicates that the large difference between the steering forces of U and O rows is responsible for the higher saturation value observed for oxygen ions than uranium ions. Steering force arises from differences in the masses and mobilities of U and O ions. It is known that O ions are naturally more mobile than U ions and that they have a liquid-like self diffusion coefficient at temperatures approximately 20% lower than the melting temperature of UO2 which is a property of a superionic conductor. Therefore, oxygen defects are more easily created during uranium IFP recombination, even at 300 K.

Bottom Line:
In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method.Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects.Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects.

ABSTRACTThe swelling that occurs in uranium dioxide as a result of radiation-induced defect ingrowth is not fully understood. Experimental and theoretical groups have attempted to explain this phenomenon with various complex theories. In this study, experimental lattice expansion and lattice super saturation were accurately reproduced using a molecular dynamics simulation method. Based on their resemblance to experimental data, the simulation results presented here show that fission induces only oxygen Frenkel pairs while alpha particle irradiation results in both oxygen and uranium Frenkel pair defects. Moreover, in this work, defects are divided into two sub-groups, obstruction type defects and distortion type defects. It is shown that obstruction type Frenkel pairs are responsible for both fission- and alpha-particle-induced lattice swelling. Relative lattice expansion was found to vary linearly with the number of obstruction type uranium Frenkel defects. Additionally, at high concentrations, some of the obstruction type uranium Frenkel pairs formed diatomic and triatomic structures with oxygen ions in their octahedral cages, increasing the slope of the linear dependence.